Systems and methods for vitrectomy

ABSTRACT

A system for conducting a vitrectomy includes: a gas source; a vitrector including a cutting mechanism that opens and closes according to a pressure at the vitrector; and a pulse-generating system receiving gas from the gas source and generating pulses at the vitrector. The pulses cause the pressure at the vitrector to vary according to a cycle, and the varying pressure at the vitrector causes the cutting mechanism of the vitrector to open and close. At a first time in the cycle, the pulse-generating system, raises the pressure at the vitrector to a maximum pressure. At a second time in the cycle, the pulse-generating system reduces the pressure at the vitrector to a minimum pressure that is greater than ambient, the pressure at the vitrector being maintained at least at the minimum pressure. The difference between the maximum pressure and minimum pressure is minimized to reduce gas consumption.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/153,912, filed on Jun. 6, 2011, and issued as U.S. Pat. No. 8,496,681on Jul. 30, 2013, the contents of which are incorporated entirely hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to vitrectomy, and more particularly, to portablesystems and methods that allow vitrectomy to be conducted in a widevariety of clinical environments.

2. Description of Related Art

Vitrectomy is the surgical removal of vitreous gel from the middle ofthe eye. Vitreous gel (also called vitreous humor) is a thick,colorless, gel-like fluid that fills the large space in the middle ofthe eye, behind the lens. Vitreous gel helps the eyeball maintain itsshape.

Removal of the vitreous gel, in some cases, provides better access tothe back of the eye for particular treatments. For example, vitrectomyprovides access for repairing or preventing traction retinal detachment,repairing very large tears in the retina, or treating severeproliferative retinopathy. Vitrectomy may also be employed to reducevision loss caused by severe or prolonged bleeding in the vitreous gel(vitreous hemorrhage).

Vitrectomy involves three functions: cutting, suction, and infusion.During vitrectomy, small incisions are made in the wall of the eye,through which various instruments are passed. In particular, the doctorinserts a working instrument, e.g., a vitrector, into the eye, cuts thevitreous gel, and suctions the vitreous gel out. A vitrector combines aguillotine style cutting mechanism with vacuum suction. When suction isinitiated, the vitreous gel is drawn through a port in the probe tip andthen severed by the cutting mechanism. In addition, saline is infusedinto the eyeball to keep the eyeball distended for treatment. Afterremoving the vitreous gel, the surgeon may treat the retina with a laser(photocoagulation), cut or remove fibrous or scar tissue from theretina, flatten areas where the retina has become detached, or repairtears or holes in the retina or macula. At the end of the surgery,saline, silicone oil, or a gas is injected into the eye to replace thevitreous gel and restore normal pressure in the eye.

Vitrectomies are typically conducted in facilities dedicated to surgicalprocedures, such as an operating room in a hospital. Special power andgas systems are generally available in such environments. These systemsare usually monitored by various computerized alarm systems. Inparticular, hospitals employ highly controlled power systems thatprovide uninterrupted electricity throughout the hospital. Furthermore,hospitals employ piped gas systems that supply pressurized oxygen,nitrous oxide, nitrogen, carbon dioxide, and/or clean outside air viapipes to operating rooms and other parts of the hospital. Becausevitrectomies are typically conducted in hospital facilities,conventional vitrectomy systems are designed to rely on the specialpower and gas systems available in such facilities. For example,conventional vitrectomy systems that employ gas-driven vitrectors can besimply connected to a gas port readily available in a hospital operatingroom to receive the pressurized gas needed for driving the vitrector.Accordingly, conventional vitrectomy systems cannot be easilyimplemented in facilities without special systems that effectivelyprovide unlimited power and pressurized gas.

SUMMARY OF THE INVENTION

In view of the foregoing, aspects of the present invention providesystems and methods that allow vitrectomy to be conducted in a widevariety of clinical environments. Advantageously, embodiments accordingto aspects of the present invention do not rely on special power and gassystems that are typically only available in special hospitalfacilities. Thus, such embodiments may be implemented in doctors'offices, which may not have access to special systems that provide powerand/or gas.

For example, aspects of the present invention use gas more efficientlyso that the vitrectomy system can use widely available and smaller gascylinders. According to one embodiment, a system for conducting avitrectomy includes: a gas source; a vitrector including a cuttingmechanism that opens and closes according to a pressure at thevitrector; and a pulse-generating system receiving gas from the gassource and generating pulses at the vitrector. The pulses cause thepressure at the vitrector to vary according to a cycle, and the varyingpressure at the vitrector causes the cutting mechanism of the vitrectorto open and close. At a first time in the cycle, the pulse-generatingsystem raises the pressure at the vitrector to a maximum pressure. At asecond time in the cycle, the pulse-generating system reduces thepressure at the vitrector to a minimum pressure that is greater thanambient, the pressure at the vitrector being maintained at least at theminimum pressure. By employing a minimum pressure greater than ambient,the difference between the maximum pressure and minimum pressure isminimized to reduce gas consumption.

Correspondingly, a method for vitrectomy includes: providing apulse-generating system with gas from a gas source; and generatingpulses from the pulse-generating system at a vitrector. The vitrectorincludes a cutting mechanism. The pulses cause the pressure at thevitrector to vary according to a cycle, and the varying pressure at thevitrector causes the cutting mechanism of the vitrector to open andclose. The act of generating pulses includes: raising, at a first timein the cycle, the pressure at the vitrector to a maximum pressure, andreducing, at a second time in the cycle, the pressure at the vitrectorto a minimum pressure that is greater than ambient, the pressure at thevitrector being maintained at least at the minimum pressure.

Additional aspects of the invention will be apparent to those ofordinary skill in the art in view of the detailed description of variousembodiments, which is made with reference to the drawings, a briefdescription of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example vitrectomy system according to aspects ofthe present invention.

FIG. 2 illustrates pneumatics for an example vitrectomy system accordingto aspects of the present invention.

FIG. 3A illustrates an example of how a vitrectomy system may drive avitrector according to aspects of the present invention.

FIG. 3B illustrates another example of how a vitrectomy system may drivea vitrector according to aspects of the present invention.

FIG. 4 illustrates aspects of a cut cycle corresponding to the exampleof FIG. 3A.

FIG. 5 illustrates example electronics corresponding to the example ofFIG. 3A.

FIG. 6 illustrates an example of how a vitrectomy system may controlaspiration according to aspects of the present invention.

FIG. 7 illustrates example electronics corresponding to the example ofFIG. 6.

FIG. 8 illustrates additional example electronics corresponding to theexample of FIG. 6.

FIG. 9 illustrates an example of how a vitrectomy system may control theair exchange according to aspects of the present invention.

FIG. 10 illustrates an example power supply for a vitrectomy systemaccording to aspects of the present invention.

FIG. 11 illustrates an example sensing system for a vial of anaspiration cassette in a vitrectomy system according to aspects of thepresent invention.

FIG. 12A illustrates an example illumination system in a vitrectomysystem according to aspects of the present invention.

FIG. 12B illustrates the relative spectral output of an LED in theexample illumination system of FIG. 12A.

FIG. 12C illustrates the relative spectral output of filtered LED lightin the example illumination system of FIG. 12A.

DETAILED DESCRIPTION

Aspects of the present invention provide systems and methods that allowvitrectomy to be conducted in a wide variety of clinical environments.Advantageously, embodiments according to aspects of the presentinvention do not rely on special power and gas systems that aretypically only available in special hospital facilities. Thus, suchembodiments may be implemented in doctors' offices, which may not haveaccess to special systems that provide unlimited power and/or gas.

Aspects of the present invention provide systems and methods for drivinga vitrector, providing aspiration, and/or handling fluid-air exchange ina vitrectomy system. Referring to FIG. 1, an example vitrectomy system100 is illustrated. Components of the vitrectomy system 100 areassembled into an apparatus defined by a housing 102. As shown in FIG.1, the housing 102 includes a handle 104 to make the vitrectomy system100 more easily portable. Advantageously, the vitrectomy system 100 issized for portability and is not restricted to a particular location orfacility. For example, the housing 102 may be approximately 0.25 m (10inches) in width by approximately 0.23 m (9 inches) in height byapproximately 0.23 m (9 inches) in depth, and may weigh approximately5.5 kg (12 lbs).

Operator controls 106, such as adjustable knobs, buttons, switches, andthe like, are provided on the exterior of the housing 102. As describedbelow, the operator controls 106 control aspects of the vitrectomysystem 100, such as the cut speed of the vitrector, the level ofaspiration, amount of illumination, etc.

In addition, a vitrector connection 108, e.g., a luer connection, isprovided on the exterior of the housing 102 to removably couple avitrector to the components in the housing 102. An air exchangeconnection 110, e.g., a luer connection, is also provided on theexterior of the housing 102 to removably couple an air exchange tubingset to the components in the housing 102. Additionally, an aspirationcassette 111 is removably received in a cassette receptacle 103 definedby the housing 102 and coupled to the components in the housing 102,e.g., via a luer. As described further below, the aspiration cassette111 provides aspiration vacuum via an aspiration cassette connection 109and receives aspirated vitreous gel and saline. Furthermore, a gassource 112 is removably coupled via a gas source connector to thecomponents in the housing 102.

Referring to FIG. 2, example pneumatics for the vitrectomy system 100are illustrated. The vitrectomy system 100 pneumatically drives alow-pressure gas-driven vitrector 210. An operator of the vitrectomysystem 100 activates a cut/aspirate mode via the controls 106 to operatethe vitrector 210. The gas source 112 provides input gas for driving thevitrector 210. The input gas is regulated at the gas source 112 to alower input pressure, e.g., approximately 80 psig, which is monitored bya pressure transducer 204.

A pulse-generating system 206 receives the gas from the gas source 112and generates pulses of gas pressure that drive the vitrector 210. Thegas pressure in the pulse-generating system 206 is further regulated toa lower pressure, e.g., approximately 16 psig. The pulses of gas cause acutting mechanism of the vitrector 210 to open and close according to acycle. The open/close action of the cutting mechanism cuts the vitreousgel when the vitrector 210 is applied to the eye. The rate at which thepulse-generating system 206 produces the pulses of gas depends on a cutspeed the operator selects for the vitrector 210 via the controls 106.In some embodiments, for example, the cut speed may be selected from arange of approximately 60 cuts per minute (cpm) to approximately 2500cpm.

Vitrector tubing 208 extends between the vitrector connection 108 shownin FIG. 1 and the vitrector 210. The vitrector tubing 208 includes afirst tube 208 a. The vitrector connection 108 couples the first tube208 a to the pulse-generating system 206 disposed at the housing 102.The first tube 208 a carries the pressure pulses from thepulse-generating system 206 to the vitrector 210.

The vitrector tubing 208 also includes a second tube 208 b. Theaspiration cassette connection 109 couples the second tube 208 b to theaspiration cassette 111 disposed in the recess 103. The second tube 208b provides a vacuum to the vitrector 210. The aspiration cassette 111 isremovably coupled to a vacuum source 222 and collects aspirant receivedfrom the vitrector 210 via the second tube 208 b.

The vitrector tubing 208, for example, may include twin-bore PVC tubinghaving a length of approximately 1 m to approximately 2 m. For a lengthof approximately 1 m, each tube 208 a, b may have an inner diameter ofapproximately 0.81 mm (0.032 inches) and an outer diameter ofapproximately 2.4 mm (0.094 inches). For a length of approximately 2 m,each tube 208 a, b may have an inner diameter of approximately 1.6 mm(0.062 inches) and an outer diameter of approximately 3.2 mm (0.125inches). The tubing length is determined in part by the desired distancebetween a patient and the housing 102. In other words, the tubing lengthshould allow the operator to extend the vitrector 210 to a patientpositioned suitably away from the housing 102. In addition, the tubinglength should provide sufficient slack so that the operator canmanipulate the vitrector 210 in space without significant restriction.

The vitrector tubing 208 may be selected from an array of standard-sizedtubing formed from medical grade materials. Specifically, from the arrayof choices, tubing of varying lengths, inner diameters, outer diameters,and durometers may be evaluated for their performance for variousconfigurations of components, e.g., pneumatic components, for thevitrectomy system 100. The vitrector tubing 108 provides sufficient gasflow to drive the vitrector 210 at the required cut speeds. The highestcut speed requires the highest gas flow in the vitrector tubing 208leading to the vitrector 210.

The gas source 112 also provides input gas for driving a pinch valvedrive 212. The pinch valve drive 212 in turn operates a pinch valve 218which controls aspiration from the eye during the procedure. An aircylinder of the pinch valve 218 closes the second tube 208 b of thevitrector tubing 208 to block flow. To allow aspiration through thesecond tube 208 b, pressure is applied to the pinch valve 218 whichcauses the air cylinder to retract. Pressure is applied to the pinchvalve 218 when the cut/aspirate mode is activated or when the aspirationcassette 111 is missing. When the cut/aspirate mode is deactivated, thepressure is relieved from the pinch valve 218 causing the pinch valve218 to pinch and close the second tube 208 b. When the cut/aspirate modeis deactivated, closure of the pinch valve 218 is delayed to allow thesecond tube 208 b to vent to atmosphere. In some embodiments, the pinchvalve 218 pinches the second tube 208 b against a surface of theaspiration cassette 111.

As described previously, the aspiration cassette 111 is coupled to thesecond tube 208 b of the vitrector tubing 208 and the vacuum source 222.The vacuum in the aspiration cassette 111 from the vacuum source 222draws aspirant from the vitrector 210 via the second tube 208 b to avial in the aspiration cassette 111. The vacuum level in the aspirationcassette 111 and the level of aspiration are controlled by an adjustablevacuum control 221 provided with the controls 106. When the cut/aspiratemode is deactivated, closure of the pinch valve 218 is delayed also toallow the aspiration cassette 111 to vent to atmosphere. When the pinchvalve 218 closes, the vacuum in the aspiration cassette 111 returns tothe level determined by the vacuum control 221. The vacuum level of theaspiration cassette 111 is monitored with a pressure transducer 228. Inoperation, the aspiration cassette 111 may be positioned substantiallylevel with the patient's eye, thereby minimizing aspirationperturbations caused by the fluid column between the aspiration cassette111 and the eye.

The vial of the aspiration cassette 111, for example, may have a volumeof approximately 25 cc. The volume of aspirant received in the vial maybe monitored with a sensor system 1100 shown in FIG. 11. In addition todetecting whether a vial contains a threshold volume of aspirant, thesensor system 1100 also detects whether the aspiration cassette 111 ismissing from the vitrectomy system 100. The sensor system 1100 includesa pair of infrared light emitting diodes (LED's) 1104 a, b that arearranged to project two separate infrared beams through the vial. A pairof transistors 1106 a, b are correspondingly positioned to receive theinfrared beams projected through the vial. When the aspiration cassette111 is missing from the vitrectomy system 100, the infrared beams do notpass through the vial and thus experience substantially no scatter.Because there is substantially no scatter, the infrared energy reachingthe transistors 1106 a, b is substantially equal to the maximum. ACASSETTE MISSING threshold corresponding to this maximum level ofinfrared energy is set. When both of the transistors 1106 a, b receiveinfrared energy that is greater than the CASSETTE MISSING threshold, theaspiration cassette 111 is considered to be missing.

When the aspiration cassette 111 is installed and present in thevitrectomy system 200, the infrared beams experience some scatter due tothe vial and the amount of infrared energy reaching the transistors 1106a, b is reduced to a level below the CASSETTE MISSING threshold. As theaspirant fills the vial, the LED's 1104 a, b are positioned so that theinfrared beams have to pass through the aspirant when the vial becomessubstantially full with aspirant. When the infrared beams pass throughthe aspirant, the infrared beams experience additional scattering andthe infrared energy reaching the transistors 1106 a, b is reducedfurther. A CASSETTE FULL threshold corresponding to this reduced levelof energy is determined. When either transistor 1106 a, b receivesinfrared energy lower than the CASSETTE FULL threshold, the vial isconsidered to be full. In general, the strength of the signal receivedby the transistors 1106 a, b indicates whether aspiration cassette 111is not present in the vitrectomy system 100 or whether the volume ofaspirant has reached a certain volume. After the vitrectomy system 100detects that the vial is full, the vitrectomy system 100 may be resetwhen it detects that the aspiration cassette 200 has been removed, orwhen the vitrectomy system 100 is reset by a POWER/RESET function viathe controls 106.

Referring again to FIG. 2, the vitrectomy system 200 generates a vacuuminternally via the vacuum source 222. The maximum vacuum, for example,may be approximately 500 mm Hg to approximately 700 mm Hg (at sealevel). The vacuum source 222 does not operate if the vitrectomy system100 is stopped due to a detected error condition, e.g., if there is aproblem with the input gas pressure or the battery charge level or ifthe aspiration cassette 111 is missing.

The vacuum source 222 is coupled to a vacuum accumulator 224. The vacuumaccumulator 224 stores system vacuum and has the capacity to maintainvacuum in the aspiration cassette 111 even when the vacuum source 222 isnot operating. The accumulator, for example, may have a volume ofapproximately 250 cc. The vacuum accumulator 224 also smoothesvariations when the vacuum source 222 is operating. The vacuum level inthe vacuum accumulator 224 is monitored with a pressure transducer 226.

The vitrectomy system 100 generates low pressure air internally via apressure source 230 to provide air exchange. The air exchange controlsinfusion pressure of an infusion fluid by adjusting a pressure in anirrigation bottle during the procedure. The air exchange may alsodeliver low pressure air to the eye during an air-fluid exchange. Thelow pressure air is directed to an accumulator 232. The accumulator 232stores pressure, e.g., at approximately 5 psig. The accumulator 232 alsosmoothes variations when the pressure source 230 is operating. Once thelow pressure source 230 provides the required pressure in theaccumulator 232, the accumulator 232 has the capacity to allow thepressure source 230 to be implemented as a supplemental vacuum source.The output pressure from the pressure source 230 is determined by an airexchange control 234 provided with the controls 106. The air exchangemay be maintained regardless of whether the cut/aspirate mode isactivated or deactivated or whether there are error conditions.Advantageously, the internal generation of low pressure air allows thevitrectomy system 100 to operate independently of any special externalair pressure source.

In one embodiment, an irrigation fluid bottle containing saline isconnected to the air exchange connection 110 via an air exchange tubeset. The air exchange tube set is a PVC tubing set intended to deliverforced air or fluid during ophthalmic surgery. The air exchange tube setincludes a three channel IV spike for insertion into the bottle. Thethree-way spike is designed to deliver air from the pressure source 230through the saline, while the dual PVC tubing administers air andsaline. The dual PVC tubing has a three-way stopcock for properconnection. The irrigation fluid bottle may be positioned so that it issubstantially level with the eye. The spike from the air exchange tubeset is coupled to the bottle and an infusion cannula is coupled to theother end of the air exchange tube set. The air exchange control 234 isset to provide a desired infusion pressure.

FIGS. 3A-B, 4, and 5 illustrate examples of how the vitrectomy system100 may drive the vitrector 210 and control the pinch valve 218. The gaspressure source 112, for instance, includes a conventional tank 312 acontaining carbon dioxide (CO₂) at a pressure of approximately 900 psig.The gas tank 312 a is removably received by the housing 102 and screwedto a gas source connector until it is firmly seated.

Employing a conventional tank to supply gas to the vitrectomy system 100allows the vitrectomy system 100 to be operated without access to aspecial gas supply system typically available in special hospitalfacilities. The vitrectomy system 100, for example, may accept gascylinders ranging in size from 12 g to 33 g. The size of such gascylinders enhances the portability of the vitrectomy system 100.Moreover, compatibility with standard smaller off-the-shelf gas tanksenhances the convenience and cost effectiveness of using the vitrectomysystem 100. Additionally, the smaller size and relatively lower cost ofthe gas cylinder allows it to be included in a convenient disposablepackage that is used for each procedure.

The pressure in the gas tank 312 a is regulated, e.g., to a pressure ofapproximately 80 psig, using a tank-mounted high pressure regulator 312b. Protection from liquid CO₂ intrusion into the high pressure regulator312 b is provided by an adapter between the gas source connector and thehigh pressure regulator 312 b. The pressure transducer 204 is used tomonitor the output from the high pressure regulator 312 b. If thepressure drops below a minimum threshold, e.g., 70 psig, the vitrectomysystem 100 shuts off.

The low pressure regulator 313 reduces the gas pressure, e.g., toapproximately 16 psig. The pulse-generating system 206 includes a valvecombination 314. The gas from the low pressure regulator 313 is directedto the valve combination 314 to generate pressure pulses for driving thevitrector 210. The pressure pulses cause the pressure at the vitrector210 to vary according to a cycle, and the varying pressure at thevitrector 210 causes the cutting mechanism of the vitrector 210 to openand close.

As shown in the example embodiment of FIG. 3A, the valve combination 314may include a two-way pressurize valve 314 a and a two-way exhaust valve314 b. The pressurize valve 314 a is coupled to the low pressureregulator 313 and to the vitrector 210. At the beginning of a cut cycle,the pressurize valve 314 a opens for approximately 6 ms to raise thepressure to a maximum of approximately 14 psig at the vitrector 210. Apressure curve 400 shown in FIG. 4 illustrates the pressure at thevitrector 210 during the cut cycle as a function of time. Acorresponding curve 410 shown in FIG. 4 indicates when the pressurizevalve 314 a remains open during a period 412 in the cut cycle. Themaximum pressure is shown as peak 402.

The exhaust valve 314 b is coupled to the vitrector 210 and toatmosphere through a muffler. During the cut cycle, the exhaust valve314 b opens for approximately 18 ms to reduce the pressure at thevitrector 210 from the maximum pressure to a minimum pressure ofapproximately 4 psig. A curve 420 shown in FIG. 4 indicates when theexhaust valve 314 b remains open during a period 422 in the cut cycle.Taken together the curves 400, 410, and 420 illustrate how the pressureat the vitrector 210 responds to the opening/closing of the pressurizevalve 314 a and the opening/closing of the exhaust valve 314 b. Theminimum pressure is shown in pressure curve 400 of FIG. 4 as trough 404.As illustrated in FIG. 4, the opening and closing of the valves 314 a, bare timed to achieve a desired pressure profile, e.g., the pressurecurve 400. With certain types of valves, the time and manner in whichthe valves 314 a, b open may be different from the time and manner inwhich the valves 314 a, b close. Thus, to account for this asymmetricbehavior in FIG. 4, for example, there is a delay of approximately 8 msbetween the time when the pressurize valve 314 a is closed and the timewhen the exhaust valve 314 b is opened.

Alternatively, as shown in the embodiment of FIG. 3B, the valvecombination 314 includes a three-way pressurize valve 314 c and atwo-way exhaust valve 314 d. The pressurize valve 314 c is coupled tothe low pressure regulator 313, the vitrector 210, and the exhaust valve314 d. The two-way valve 314 d is coupled to the pressurize valve 314 cand to atmosphere through a muffler. At the beginning of the cut cycle,the pressurize valve 314 a opens for approximately 6 ms to raise thepressure to a maximum of approximately 14 psig at the vitrector 210.When the pressurize valve 314 c closes, the vitrector 210 is coupled tothe exhaust valve 314 d via the pressurize valve 314 c. Thus, theexhaust valve 314 d opens at the same time that the pressurize valve 314c closes. The exhaust valve 314 d couples the vitrector 210 toatmosphere via a muffler. The exhaust valve 314 d remains open forapproximately 18 ms and reduces the pressure at the vitrector 210 fromthe maximum to a minimum of approximately 4 psig.

In general, the embodiments of the pulse-generating system 206 shown inFIGS. 3A-B include a first valve and a second valve, where the firstvalve and the second valve are operable to raise and reduce the pressureat the vitrector 210 according to a cycle that drives the vitrector 210.At a first time in the cycle, the pulse-generating system 206 raises thepressure at the vitrector 210 to a maximum pressure. At a second time inthe cycle, the pulse-generating system 206 reduces the pressure at thevitrector 210 to a minimum pressure that is greater than ambient.Instead of exhausting the gas from the vitrector 210 and in thevitrector tubing 208 until the pressure reaches ambient, the pressure atthe vitrector 210 and in the vitrector tubing 208 is maintained at aminimum pressure that is greater than ambient. As a result, to raise thepressure to the maximum pressure during the cut cycle, the gas source112 is required to supply less gas than would otherwise be required ifthe pressure at the vitrector 210 and in the vitrector tubing 108 werepermitted to drop to ambient. Accordingly, the embodiments use gas moreefficiently during the cut cycle and deplete the supply gas from the gassource 112 more slowly.

The efficient use of gas by the vitrectomy system 100 is particularlyadvantageous if the vitrectomy system is employed with gas tanks thatare 12 g to 33 g in size, for example. The embodiments shown in 3A-B canprovide approximately 6 minutes to approximately 19 minutes of vitrectoroperation when gas pressure in the system is kept at or above a minimumthreshold greater than ambient. The operating time depends on the cutspeed of the vitrector 210. For example, a 16 g cylinder tank mayprovide approximately 8 to approximately 9 minutes of vitrectoroperation when the vitrector 210 is operated at 1200 cpm. Of course, insome embodiments, an adapter is available to allow the vitrectomy system100 to be used with a special gas supply system, if available. Thespecial gas supply system effectively extends the operating time of thevitrectomy system 100 indefinitely.

Although the embodiments described previously raise the pressure to amaximum of approximately 14 psig at the vitrector 210 and reduce thepressure to a minimum pressure of approximately 4 psig, the maximum andminimum pressures during the cut cycle may be different in otherembodiments. The maximum and minimum pressures are selected according tothe pressures required to operate the vitrector 210. For instance, themaximum pressure during the cut cycle should be sufficient to cause thecutting mechanism of the vitrector 210 to close completely and toachieve the required cut speed. Higher cutting speeds can be achievedwith greater spring force in the cutting mechanism of the vitrector 210and correspondingly higher applied pressure. To promote efficient use ofgas, the maximum pressure is preferably the lowest pressure required forthe cutting mechanism to close completely. The pressure required for thecutting mechanism to close completely depends partially on the forcerequired to overcome the friction as well as the spring force associatedwith the cutting mechanism of the vitrector 210.

Meanwhile, the minimum pressure during the cut cycle should besufficiently low to cause the cutting mechanism to open completely. Topromote efficient use of gas further, the minimum pressure is preferablythe highest possible pressure that still ensures that the cuttingmechanism of the vitrector 210 opens completely. In general, embodimentsemploying the valve combination 314 described above achieve moreefficient use of gas by minimizing the difference between the maximumand minimum pressures during the cut cycle. In some embodiments, thegreatest efficiencies are achieved with a pressure difference ofapproximately 8 psig to approximately 10 psig.

The pressurize valve and the exhaust valve are opened for the necessaryperiods of time to achieve a maximum pressure and a minimum pressure,respectively. The periods of time, as well as the minimum and maximumpressures, depend on the structure and configuration of the valvecombination 314 and the vitrector 210. Typically, the pressurize valvedoes not remain open longer than half the time for the cut cycle,because it takes more time for the exhaust valve to reduce the pressureto the minimum than it takes for the pressurize valve to raise thepressure to the maximum. To optimize air flow in the vitrectomy system100, the pulse-generating system 206 may be adjusted to deliver amaximum pressure that is sufficient to provide the largest cut speedduring the shortest amount of time required to open the pressurizevalve.

In addition to achieving more efficient use of gas and extending theoperating time provided by the gas source 112, it has been discoveredthat embodiments that minimize the difference between the maximum andminimum pressures during the cut cycle also provide additionaladvantages. In particular, the embodiments reduce the noise associatedwith operating the vitrectomy system 100. Noise is a distraction duringthe procedure. Conventionally, mufflers are used in vitrectomy systemsto reduce noise, but the noise is generally difficult to muffle due tothe pulsing nature of the vitrectomy system 100. Changing the minimumand maximum pressures as described above, however, yields greater noisereduction, because less air is being moved during each cut cycle.

Furthermore, the embodiments also reduce the amount of stiffness thatthe gas pressure causes in the vitrector tubing 208. The stiffnessincreases with greater average operation pressure in the vitrectortubing 208. The vitrector 210, however, is easier to manipulate when thevitrector tubing 208 remains more flexible.

In addition, when the difference between the maximum and minimumpressures is larger and the vitrector tubing 208 is stiffer, thevitrector tubing 208 has a greater tendency to bounce or vibrate andtransfer disturbances to the operator's hand holding the vitrector 210.Accordingly, reducing the stiffness of the vitrector tubing 208 givesoperators a greater tactile feel and steadier control of the vitrector210.

As discussed previously, the vitrector tubing 108 must providesufficient gas flow to drive the vitrector 210 at the required cutspeeds. The vitrectomy system 100, however, also minimizes consumptionof the gas from the gas source 112. A longer tube with larger innerdiameter provides a higher gas flow rate and results in more gasconsumption. Conversely, a shorter tube with smaller inner diameterprovides a lower gas flow rate and results in less gas consumption. Asdiscussed previously, the length of the vitrector tubing 208 isdetermined by practical considerations. For example, the length of thevitrector tubing 208 is not shorter than 1 meter. Thus, for a givenlength of the vitrector tubing 208, the inner diameter may be reduced tothe point where the gas flow cannot drive the vitrector 210 at therequired cut speeds. In addition to minimizing gas consumption, thesmaller inner diameter results in less stiffness in the vitrector tubing210.

Referring again to FIG. 3B, the pinch valve drive 212 also employs avalve combination 322 to control the pressure that drives the pinchvalve 218. The valve combination 322 includes a pressurize valve 322 aand an exhaust valve 322 b. The pressurize valve 322 a is connected tothe low pressure regulator 313 and an air cylinder for the pinch valve218. The air cylinder of the pinch valve 218 may be a reverse acting(spring extended) air cylinder that aids in closing the second(aspiration) tube 208 b of the vitrector tubing 208. In operation, thepressurize valve 322 a opens, e.g., for approximately 100 ms, to openthe pinch valve 218. The exhaust valve 322 b is connected to the aircylinder of the pinch valve 218 and to atmosphere through a muffler. Theexhaust valve 322 b opens, e.g., for approximately 100 ms, to close thepinch valve 218.

FIG. 5 illustrates example electronics for driving the vitrector 210 andcontrolling the pinch valve 218 for a vitrectomy system 100 that employsthe valve combination 314 shown in FIG. 3A. The vitrector 210 iscontrolled by an illuminated toggle switch 502 and an adjustablepotentiometer 522 provided with the controls 106. The adjustablepotentiometer 522 determines the cut rate for the vitrector 210. Theswitch 502 is operated to alternate between activating and deactivatingthe cut/aspirate mode and to respectively enable or disable cutting withthe vitrector 210. When the cut/aspirate mode is deactivated, theaspiration system is also returned to atmosphere with the pinch valve218 closed. The switch 502 is illuminated with a combination RED/GREENLED 504. The LED is illuminated RED when the available gas pressurefalls below a minimum threshold, e.g., approximately 70 psig. The LED isilluminated GREEN when the cut/aspirate mode is activated. The LED isnot illuminated when the cut/aspirate mode is deactivated.

Operation of the vitrectomy system 100 is enabled when the switch 502 isswitched to activate the cut/aspirate mode. Although embodiments of thevitrectomy system 100 may employ a microprocessor, the embodiment shownin FIG. 5 employs discrete integrated circuit (IC) functional blocks(logic gates, flip-flops, multivibrators, etc.). In particular, thevitrectomy system 100 employs a multivibrator chain 506 that determinesoperation of the vitrector 210 according to the cut cycle. Thus, whenthe switch 502 is switched to ON, the switch pull-up resistor 504 isgrounded and the RESET signal is removed from the multivibrator chain506. The monostable multivibrator 506 a is triggered to open the two-waypressurize valve 314 a. Triggering the monostable multivibrator 506 acauses the pressure to increase at the vitrector 210 and the cuttingmechanism to close. After the pressurize valve 314 a is closed for therequired amount of time, e.g., approximately 6 ms, the next monostablemultivibrator 506 b is triggered. As described previously, there is adelay, e.g., approximately 8 ms, between the time when the pressurizevalve 314 a is closed and the time when the exhaust valve 314 b isopened. After the delay time expires, the final monostable multivibrator506 c in the chain 506 is triggered. The monostable multivibrator 506 copens the two-way exhaust valve 314 b and causes the cutting mechanismof the vitrector 210 to open. The exhaust valve 314 b closes after therequired amount of time, e.g., approximately 18 ms. When thecut/aspirate mode is activated with the switch 502, an astablemultivibrator 506 d runs at the cut rate selected for the vitrector 210.The astable multivibrator 506 d triggers the monostable multivibrator506 a controlling the pressurize valve 314 a and repeats the cut cycle.

During the procedure, the tip of the vitrector 210 is placed at an areaof vitreous gel requiring removal. The cut/aspirate mode may beactivated through the toggle switch 502 or a foot pedal. When thecut/aspirate mode is activated, the vitrectomy system 100 cuts andaspirates at the levels determined by the adjustable cut rate control522 and the adjustable vacuum control 221. Once the desired volume ofvitreous gel has been removed, the cut/aspirate mode is deactivated bypressing the toggle switch 502 again or by releasing the foot pedal.

When the cut/aspirate mode is deactivated, the cutting mechanism of thevitrector 210 stops immediately. In addition, the monostablemultivibrator 506 c controlling the exhaust valve 314 b is triggered toforce the cutting mechanism of the vitrector 210 to open.

FIG. 6 illustrates an example of how the vitrectomy system 100 maycontrol aspiration. The vacuum source 222 includes a vacuum pump 602,such as a 6 VDC diaphragm pump. The exhaust for the vacuum pump 602 isconnected to a muffler. The vacuum pump 602 is activated when the vacuumat the vacuum accumulator 224 drops below a vacuum pressure activationthreshold. The vacuum pressure activation threshold, for example, is setto a pressure approximately 25 mmHg above the aspiration level specifiedby the adjustable vacuum control 221. The vacuum level in the vacuumaccumulator 224 is monitored with a pressure transducer 226. As shown inthe electronics of FIG. 7, a vacuum level determined by a sensor 702 iscompared to the vacuum pressure activation threshold. The vacuum leveldetermined by the sensor 702 is also compared to a vacuum pressuredeactivation threshold. The vacuum pump 702 is stopped when the vacuumin the accumulator meets the vacuum pressure deactivation threshold (orwhen an error condition occurs).

As illustrated further in FIG. 6, a proportional valve 612 a and atwo-way valve 612 b control the vacuum level in the aspiration cassette111. The proportional valve 612 a is coupled to the vacuum accumulator224 and the aspiration cassette 111 (through a hydrophobic filter 616).The proportional valve 612 is operated to lower the pressure in theaspiration cassette 111 to the value determined by the vacuum control221. The proportional valve 612 a is opened by an amount that minimizesovershoot and vacuum response time. The vacuum level of the aspirationcassette 620 is monitored with a pressure transducer 228. The two-wayvalve 612 b is coupled to the aspiration cassette 111 and to atmosphere.The two-way valve 612 b is operated to raise the pressure in theaspiration cassette 111.

FIG. 8 illustrates example electronics for controlling the aspiration inthe vitrectomy system 100. The operator controls the aspiration level bythe adjustable vacuum control 221, which includes a potentiometer 802 ora linear foot pedal 804. The adjustment allows the vacuum to be set, forexample, from approximately 25 mmHg to approximately 500 mmHg. Thesetting is compared with a vacuum sensor 806 that is coupled to theaspiration cassette 111. The vacuum sensor 806 may be similar to thesensor 702 that monitors the vacuum of the vacuum accumulator 224. Inboth cases, the sensors have a 4 VDC range which equates to 15 psig(approximately 775.7 mmHg).

If the vacuum level in the aspiration cassette 111 is less than theaspiration level set by the operator (and there is no error condition orcassette venting in progress), the two-way valve 612 a opens and couplesthe aspiration cassette 111 to the vacuum accumulator 224. Theaspiration cassette 111 passes air to the vacuum accumulator 224 causinga reduction of pressure in the aspiration cassette 111. Once the vacuumlevel in the aspiration cassette 111 matches the aspiration level set bythe user, the two-way valve 612 a closes and decouples the aspirationcassette 111 from the vacuum accumulator 224.

The aspiration cassette 111 is coupled to atmosphere when the vacuumlevel in the aspiration cassette 111 is higher than an aspirationcassette vacuum threshold. If the vacuum level in the aspirationcassette 111 is greater than the aspiration cassette vacuum threshold orventing of the aspiration cassette 111 is in progress, the two-way valve612 b allows the aspiration cassette 111 to attain atmospheric pressure.

A second mechanism controls venting of the aspiration cassette 111 toatmosphere when the cut/aspirate mode is deactivated or an alarmcondition occurs. In this case, a D latch causes the proportional valve612 a to close the connection between the aspiration cassette 111 andthe vacuum accumulator 224, while the two-way valve 612 b connects theaspiration cassette 111 to atmosphere. Once the aspiration cassette 111meets the CASSETTE AT ATMOSPHERE threshold, the D latch is reset and thepinch valve 218 pinches the tube 208 b closed. Resetting of the D latchalso allows the proportional valve 612 a to couple the aspirationcassette 111 to the vacuum accumulator 224 so that the aspirationcassette 111 can be driven subsequently to the vacuum level set by theoperator.

The pinch valve 218 is opened for normal aspiration when thecut/aspirate mode is activated (and no error conditions are present).The pinch valve 218 is also opened if the aspiration cassette 111 isremoved from the vitrectomy system 100 to install a replacementcassette.

FIG. 9 illustrates an example of how the vitrectomy system 100 mayhandle air exchange. The pressure source 230 includes a pressure/vacuumpump 902, such as a 6 VDC diaphragm pump. Through two three-way valves904 a, b, the pressure/vacuum pump 902 can be employed for air exchangefunction as well as aspiration function. The three-way valves 904 a, bare configured so that when they are closed, the pressure/vacuum pump902 delivers pressure for air exchange function. Conversely, when thethree-way valves 904 a, b are open, the pressure/vacuum pump 902delivers vacuum for aspiration function. Priority is given to the airexchange function. The pressure/vacuum pump 902 is operated to increasethe pressure at the accumulator 232 when the pressure drops below athreshold, e.g., approximately 5 psig. The pressure at the accumulator232 is monitored with the pressure transducer 236.

As FIG. 9 illustrates further, a proportional valve 912 a and a two-wayvalve 912 b are employed to control the output level of the air exchangepressure. The proportional valve 912 a is coupled to the accumulator 232and the output 110 (through a hydrophobic filter). The proportionalvalve 912 a is operated to raise the output pressure to the value set bythe air exchange control 234. Once the output pressure reaches the setvalue, the proportional valve 912 a is closed or its opening is adjustedto match a constant flow load. The amount that the proportional valve912 a opens is controlled to minimize overshoot and pressure responsetime. The output pressure level is monitored by a pressure transducer916. Meanwhile, the two-way valve 912 b is coupled to the output 914 andto atmosphere and is operated to lower the output pressure.

Referring to FIG. 10, the vitrectomy system 100 employs a rechargeablebattery 1002. The vitrectomy system 100 can be coupled to an externalpower source to recharge the rechargeable battery 1002. It is notnecessary, however, for the vitrectomy system 100 to be completelycharged to use it. When connected to the external power source, thevitrectomy system 100 operates with the power from the external powersource while the rechargeable battery 1002 is recharged. Advantageously,the rechargeable battery 1002 allows the vitrectomy system 100 to beoperated without access to an external power source. The rechargeablebattery 1002 enhances the portability of the vitrectomy system 100.Moreover, the rechargeable battery 1002 can be conveniently recharged byconnecting the vitrectomy system to a conventional electrical wallsocket.

It is also not necessary to turn on the vitrectomy system 100 for thebattery 1002 to charge. If the vitrectomy system 100, however, is turnedon, a power indicator shows the state of battery charge. The powerindicator illuminate via a GREEN and a RED LED. If the power indicatorilluminates RED, there may not be enough charge to complete theprocedure. In some cases, the power indicator may not illuminate due toa depleted battery 1002. Three comparators 1004 a-c are utilized toprovide a visual indication of the battery run time. The firstcomparator 1004 a determines whether the estimated battery run timeexceeds 30 minutes. The second comparator 1004 b determines whether theestimated battery run time exceeds 20 minutes. These two comparatoroutputs pass through a transparent latch 1006 that is enabled when thevitrectomy system 100 is operating from the rechargeable battery 1002,and is selectively enabled during certain periods as the rechargeablebattery 1002 is charging. The outputs from the latch 1006 are coupled toan astable multivibrator 1008 that is forced into reset when the batteryvoltage exceeds 30 minutes (GREEN power indicator) and is forced intotrigger when the battery voltage falls below 20 minutes (RED powerindicator). Between 20 minutes and 30 minutes, the multivibrator 1008 isallowed to run at a 60 Hz rate (combining RED and GREEN to provide aYELLOW power indicator). The third comparator 1004 c determines whetherthe estimated battery run time is less than 3 minutes, in which case analarm is provided to alert the operator.

While some components of the vitrectomy system 100 may be reusable,other components, such as the gas cylinder tank 202a, the vitrector 210,the vitrector tubing 208, the aspiration cassette 111, irrigationtubing, etc., may be disposable and provided for use in a singleprocedure. The disposable components may be conveniently packaged in asingle disposable pack, e.g., with Tyvek® material. The vitrector 210,the vitrector tubing 208, the aspiration cassette 111, and theirrigation tubing are disposed in a manner consistent with othercontaminated waste.

The vitrectomy system 100 may also include a disposable endoilluminator.The illumination level may be adjusted to a desired amount with thecontrols 106. The light output is reduced when a probe of theendoilluminator is not in the eye to reduce the possibility of damage tothe fibers of the endoilluminator and to increase battery life.

FIG. 12A illustrates an example illumination system 1200 for providinglight to the endoilluminator via an output connector receptacle 1202.The output connector receptacle accepts an optical fiber connector. Inone embodiment, for example, the optical fiber connector may beconstructed from a 19-gauge hypodermic needle (approximately 1.1 mm(0.42 inches) in diameter) having a length of approximately 6.35 cm (2.5inches). The light source in the illumination system 1200 includes anLED 1205. As shown in FIG. 12A, the LED 1205 is provided on a printedcircuit board (PCB) 1203. For example, the LED 1205 may be a blue-violetLED (approximately 435 nm) with a phosphor coating to convert a portionof the light to broadband light of longer wavelength that appears white.The light emitting portion of the LED 1205 may be approximately 2 mm².FIG. 12B illustrates the relative spectral output of the LED 1205.

The LED 1205 includes a flat window that protects the diode surface butdoes not significantly affect the radiation pattern of the LED 1205. Alens 1206 gathers light from the LED 1205. The lens 1206, for example,is a plastic hybrid asphere with a numerical aperture (NA) of 0.63. Thelens 1206 has a combination of aspheric and diffractive surfaces toreduce both chromatic and spherical aberration common to traditionallens designs. The light emitting surface of the LED 1205 is placed atthe focal point of the lens 1206. Light exiting the lens 1206 isgenerally collimated. An absorptive filter 1208 is placed in the lightcolumn to tailor the spectral distribution. The filter 1208, forexample, may be formed from Schott GG435 glass that is approximately 6mm thick. The filter 1208 works in concert with the specific spectraloutput of the LED 1205. In other words, the specification for the filter1208 may depend on the LED output spectrum. Consideration is given tocolor, phototoxicity, and luminous efficacy of the output spectrumduring specification of the filter 1208. As a reference, the unfilteredoutput from the LED 1205 may measure 0.30, 0.28 CIE chromaticity, 625lumens per hazardous watt, and 271 lumens per watt, while the filteredoutput may measure 0.36, 0.39 CIE chromaticity, 1951 lumens perhazardous watt, and 363 lumens per watt. All of which are much moredesirable for an ophthalmic light source. FIG. 12C illustrates therelative spectral output of the filtered LED light.

A second lens 1210, which is substantially similar to the collimatinglens 1206, gathers the filtered light and focuses it to an image of theLED 1205. The proximal end of an optical fiber 1202 is placed at thefocal point of the second lens 1210 to guide the light to the surgicalfield.

Output power from the LED 1205 may be sensitive to temperature. Asoperating current is increased, die temperature also tends to increase.As die temperature increases, the output light level at a fixed drivecurrent tends to decrease relative to the same drive current at lowertemperature. The PCB 1203 to which the LED 1205 is mounted maximizesheat removal from the LED 1205. The board, for example, may be formedfrom approximately 0.38 mm (0.015 inch) thick fiberglass with coppertraces (for signals and power) located on the top (LED) side of theboard. Plated through-holes provide attachment points to solder the PCB1203 to a copper base plate 1209, e.g., having a thickness ofapproximately 3.175 mm (0.125 inch). The copper base plate 1209 has acopper post of approximately 0.38 mm (0.015 inch) that protrudes througha hole in the PCB 1203 to provide direct contact between the centralcontact of the LED 1205 and the copper base plate 1209. The centralcontact of the LED 1205 is provided to allow heat to be conducted awayfrom the light emitting die. This configuration allows the heat from theLED 1205 to quickly spread across a large area with minimal thermalresistance. The copper base plate 1209 is mounted to a Peltier cooler1207. The Peltier cooler 1207 pumps heat from the copper base plate 1209to an aluminum post 1204 to which the cooler 1207 is mounted. The amountof heat that is pumped relates to the amount of drive current given tothe Peltier cooler 1207. The aluminum post 1204 is mounted to amulti-finned aluminum heat sink. The heat sink efficiently transfersheat to cooling air that is blown across it by that fan that is alsomounted onto the heat sink. A thermistor is mounted on the PCB 1203 witha direct connection to the copper post to monitor LED temperature.Control electronics use the signal from the thermistor to adjust thePeltier cooler 1207 current and control temperature of the LED 1205.

As described above, aspects of the present invention provide systems andmethods for driving a vitrector, providing aspiration, and handlingfluid-air exchange in a vitrectomy system without relying on specialpower and gas systems. For example, aspects of the present invention usegas more efficiently so that the vitrectomy system can use widelyavailable and smaller gas cylinders. Thus, corresponding embodiments areportable and may be used in a variety of clinical environments.

While the present invention has been described in connection with anumber of exemplary embodiments, and implementations, the presentinventions are not so limited, but rather cover various modifications,dimensions, shapes, and equivalent arrangements. Other implementationsof the invention will be apparent to those skilled in the art fromconsideration of the specification and practice of the inventiondisclosed herein. Various aspects and/or components of the describedembodiments may be used singly or in any combination. It is intendedthat the specification and examples be considered as exemplary only. Forexample, although embodiments described above may be indicated forvitreous biopsy and vitreous taps, aspects of the present invention maybe applied in other procedures, such as floaterectomies and bloodremoval.

What is claimed is:
 1. A system for conducting a vitrectomy, comprising:a gas source; a vitrector including a cutting mechanism that opens andcloses according to a pressure and a spring force at the vitrector; anda pulse-generating system receiving gas from the gas source andgenerating pulses at the vitrector, the pulses causing the pressure atthe vitrector to vary according to a cycle, the varying pressure and aspring force at the vitrector causing the cutting mechanism of thevitrector to open and close, wherein, at a first time in the cycle, thepulse-generating system, generates a pulse and raises the pressure atthe vitrector to a maximum pressure, to close the cutting mechanism, andat a second time in the cycle, the pulse-generating system reduces thepressure at the vitrector to a minimum pressure that is greater thanambient, the pressure at the vitrector being maintained at least at theminimum pressure and the spring force causing the cutting mechanism toopen.
 2. The system of claim 1, wherein the pulse-generating systemincludes a first valve and a second valve, the first valve and thesecond valve being operable to raise and reduce the pressure at thevitrector according to the cycle.
 3. The system of claim 1, wherein thepulse-generating system includes a first two-way valve and a secondtwo-way valve, the first two-way valve being coupled to the gas sourceand to the vitrector, and the second two-way valve being coupled to thevitrector and to atmosphere.
 4. The system of claim 3, wherein, at thefirst time in the cycle, the first two-way valve opens for a firstperiod of time to raise the pressure at the vitrector to the maximumpressure, the maximum pressure being equal to a first threshold pressureto close the cutting mechanism of the vitrector, and, at the second timein the cycle, the second two-way valve opens for a second period of timeto reduce the pressure at the vitrector to the minimum pressure, theminimum pressure being equal to a second threshold pressure to allow thecutting mechanism to open.
 5. The system of claim 4, wherein the secondtwo-way valve opens after a delay after the first two-way valve closes.6. The system of claim 1, wherein the gas source is a portable gascanister having a size of approximately 12 g to 33 g.
 7. The system ofclaim 1, further comprising tubing that directs gas to the vitrector,the tubing being maintained at least at the minimum pressure.
 8. Thesystem of claim 7, wherein the tubing has a given length, and for thegiven length, the tubing has a selected inner diameter that minimizesair flow while allowing the vitrector to operate at a range ofselectable cut speeds.
 9. A method for vitrectomy, comprising: providinga pulse-generating system with gas from a gas source; and generatingpulses from the pulse-generating system at a vitrector, the vitrectorincluding a cutting mechanism, the pulses causing the pressure at thevitrector to vary according to a cycle, the varying pressure and aspring force at the vitrector causing the cutting mechanism of thevitrector to open and close, the act of generating pulses including:raising, at a first time in the cycle, the pressure at the vitrector toa maximum pressure by the generation of a pulse by the pulse-generatingsystem to close the cutting mechanism, and reducing, at a second time inthe cycle, the pressure at the vitrector to a minimum pressure that isgreater than ambient, the pressure at the vitrector being maintained atleast at the minimum pressure and the spring force causing the cuttingmechanism to open.
 10. The method of claim 9, wherein thepulse-generating system includes a first valve and a second valve, thefirst valve and the second valve being operable to raise and reduce thepressure at the vitrector according to the cycle.
 11. The method ofclaim 9, wherein the pulse-generating system includes a first two-wayvalve and a second two-way valve, the first two-way valve being coupledto the gas source and to the vitrector, and the second two-way valvebeing coupled to the vitrector and to atmosphere.
 12. The method ofclaim 11, wherein the act of generating pulses includes: at the firsttime in the cycle, opening the first two-way valve for a first period oftime to raise the pressure at the vitrector to the maximum pressure, themaximum pressure being equal to a first threshold pressure to close thecutting mechanism of the vitrector, and at the second time in the cycle,opening the second two-way valve for a second period of time to reducethe pressure at the vitrector to the minimum pressure, the minimumpressure being equal to a second threshold pressure to allow the cuttingmechanism to open.
 13. The method of claim 12, wherein the act ofgenerating pulses includes opening the second two-way valve opens aftera delay of approximately 8 ms after the first two-way valve closes. 14.The method of claim 9, wherein the gas source is a portable gas canisterhaving a size of approximately 12 g to 33 g.
 15. The method of claim 9,wherein the pulse generating system includes tubing that directs gas tothe vitrector, the tubing being maintained at least at the minimumpressure.
 16. The method of claim 15, further comprising wherein thetubing has a given length, and for the given length, the tubing has aselected inner diameter that minimizes air flow while allowing thevitrector to operate at a range of selectable cut speeds.
 17. A systemfor conducting a vitrectomy, comprising: a gas source; a vitrectorincluding a cutting mechanism that opens and close according to apressure at the vitrector; and a pulse-generating system receiving gasfrom the gas source and generating at the vitrector, the pulses causingthe pressure at the vitrector to vary according to a cycle, the cycleincluding only a single pulse, the varying pressure at the vitrectorcausing the cutting mechanism of the vitrector to open and close,wherein, at a first time in the cycle, the pulse-generating systemgenerates the single pulse and raises the pressure at the vitrector to amaximum pressure, and at a second time in the cycle, thepulse-generating system reduces the pressure at the vitrector to aminimum pressure that is greater than ambient, the pressure at thevitrector being maintained at least at the minimum pressure.